In sub-sea production, e.g., electrically operated apparatuses on sea floor are supplied power from sea- or land-based host facilities via an umbilical. AC-voltage is conducted via one or more single-phase conductors to submerged process control equipment, pumping and compression equipment, transformers and motors etc., controlling the production for example via valves and actuators on Christmas trees that sit on gas or oil wells and govern the extraction and injection of gas and liquids. Electrical power is connected into the sub-sea equipment via sub-sea connectors, arranged to establish connection and to conduct power from the power cables into transformers, motors, switchgears, VSDs, etc.
In sub-merged applications it is for several reasons indispensable that the cable termination and lead-through of conductors is protected from ingress of water. Considerable pressures prevailing at operational water depths down to and below 1,000 m necessitates a structure that is adapted to existing nominal pressures and differential pressures over seals included in the cable termination assembly. Electrical appliances and power cable terminations for sub-sea use are thus usually enclosed in housings that are filled with a dielectric and incompressible medium, such as oil, in order to withstand the ambient pressure from the sea. Compensation for pressure variations and pressure differences between the housing interior and exterior is called for, and may be facilitated through expandable communicating vessels, e.g., arranged on the housing interior and communicating with the housing exterior.
The power demand of the sub-sea appliances referred to is satisfied through the conduction of high and medium voltage AC-current through power cables typically comprising a central conductor including copper or aluminum surrounded by a solid insulator of cross-linked polyethylene. Additional layers of conductive or semi-conductive screens and insulating layers are usually arranged about the conducting core, all covered by an outer insulating cover such as a polyethylene sheath. A power cable in sub-sea applications may be dimensioned for conducting voltages in the order of up to and above 100 kV over substantial distances in the sea, having a conductor cross-sectional area adapted for current ratings of several hundreds of amperes, such as a conductor section of 25-2500 mm2, e.g., with a capacity ranging to 3000 A. Considering the operational depths, power cables for sub-sea use also need to be designed to withstand substantial external hydrostatic pressures so as to prevent the ingress of water into the cable structure. A single-phase conductor is shown schematically in FIG. 2 of the drawings attached hereto, comprising a conductive core C, a solid insulator P, an outer sheath S, and appropriate additional layers according to current praxis not further illustrated.
The termination of a power cable for submerged use is designed for establishing electrical contact with power consumers or power suppliers in a liquid-insulated environment. In connecting mode the cable termination penetrates into a connection chamber filled with dielectric liquid and housing a contact that mates with the plugged-in cable termination.
Liquid is conventionally used for insulation of the unsheathed conductor in high and medium voltage cable terminations above sea level, alone or in combination with solid dielectric materials. Beside the risk of leakage of dielectric liquid, a liquid insulated cable termination system in a sub-sea application is more vulnerable to ingress of water and provides less protection against over potentials and partial discharge, than a system relying on correspondingly sized solid insulators, e.g. Also, heat expansion of the dielectric liquid must be designed for, resulting in additional structural volume of a liquid insulated system.
In this context, the plug-in termination of the present invention is referable to the “dry” connectors relying on solid insulation materials in all areas where field strength is high, in contrast to the “wet” connectors, essentially or supplementary relying on dielectric liquids for electrical insulation. The dry mateable connector according to the present invention is useful under all conditions where electrical contact must be established below water level, thus not exclusively in sub-sea production but also in mining, e.g., or other environments where water may be present.
A plug-in power cable termination for sub-sea use is previously known from WO 99/34495. Three single-phase conductors are jointly guided into an outer pressure-compensated housing filled with dielectric liquid, providing a first barrier towards the ambient sea water. Within the outer housing, the three conductors are separated such that each single-phase conductor is terminated in a separate, inner pressure-compensated housing filled with dielectric liquid and providing a second barrier. Each conductor is secured in the associated inner housing by the conductor end being arrested in a terminating area. The unsheathed conductor end is electrically connected within the inner housing to a conductor pin which reaches through a wall of dielectric material. The dielectric wall is mounted in the leading end of the inner housing, said wall providing the liquid and gas tight barrier between the cable termination and attached apparatus. A couple of ceramic rings are mounted in a forward end of the barrier wall, concentrically about the projecting conductor pin, and operative to increase the creeping current distance between conductor pin and ground potential in the housing.